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What are reference intervals or reference ranges

You may have noticed that some of your test results are shown in your report as a comparison against a set of numbers called reference interval or reference ranges. This is the interval or range of test results that has been deemed ‘normal’ for the general population – most healthy people can be expected to have results that fall within this interval.  

How do we determine what is normal for a diagnostic test? The first step in defining a reference interval is to define the ‘healthy’ population. Reference intervals are established by taking the results of large numbers of healthy people and calculating what is typical for them. Factors such as age, sex and ethnicity can affect results, so often a series of reference intervals is established. 

The reference interval for a particular laboratory test is expressed as the average value for the ‘normal’ population group together with the variation around that value (plus or minus 2 standard deviations from the average). In this way, intervals represent the values found in 95% of individuals in the reference group. A consequence of this approach is that even in a ‘normal’ population a test result is outside the reference range in 5% of cases (1 in 20).

It’s important to realise that having a result that is marked ‘high’ or ‘low’ and is outside the reference interval is not necessarily something you need to be alarmed about.  Your result should be considered within the context of your personal circumstances and with the benefit of your own and your doctor's knowledge of your past medical and personal history, together with the results of any other investigations performed. For more about interpretation of your results and reference ranges see Reading your results.
Harmonised reference intervals
For most tests on the Pathology Tests Explained website, we do not show the reference interval or range. This is for several reasons, including the fact that different laboratories use different kinds of equipment and different kinds of testing methods. This means they have to establish their own intervals and provide your test result with an accompanying reference interval that is appropriate to the laboratory where your sample is tested.

Scientists and pathologists have recognised for some time that having multiple reference intervals, albeit sometimes with a small variation in the numerical limits of the interval, is not ideal and a potential cause of confusion. Accordingly, the pathology profession has been engaged in a project that will lead to a single reference interval for certain tests, no matter in which laboratory the sample is tested. The tests where this is possible are shown below. For some of these tests, there will be age-related intervals which means specific intervals for adults and for children (paediatric reference intervals) some of which vary by age.
Tests with harmonised reference intervals (Adults)
Test Reference Interval
Sodium 135–145 mmol/L
Potassium 3.5–5.2 mmol/L
Chloride 95–110 mmol/L
Bicarbonate 22–32 mmol/L
Creatinine Male - 60–110 µmol/L
Female - 45–90 µmol/L
Calcium 2.10–2.60 mmol/L
Calcium corrected for albumin 2.10–2.60 mmol/L
Phosphate 0.75–1.50 mmol/L
Magnesium 0.70–1.10 mmol/L
Lactate dehydrogenase 120–250 U/L
Alkaline phosphatase 30–110 U/L
Total protein 60–80 g/L
Bilirubin 1 – 20 µmol/L
Creatine kinase Male (18-60 years) 45 – 250 U/L
Male (60 -120 years) 40 – 200 U/L
Female 30 – 150 U/L
Alanine aminotransferase Male 5 - 40 U/L
Female 5 - 35 U/L
Aspartate aminotransferase Male 5 – 35 U/L
Female 5 – 30 U/L
Gamma glutamyltransferase Male 5 – 50 U/L
Female 5 – 35 U/L
Lipase 10 – 60 U/L
Tests with harmonised reference intervals (Paediatrics)
Test Reference Interval
Sodium 0 day to <1 week 132–147 mmol/L
1 week to 18 years 133–144 mmol/L
Potassium 0 day to <1 week 3.8–6.5 mmol/L
1 week to 26 weeks 4.2–6.7 mmol/L
26 weeks to 2 years 3.9–5.6 mmol/L
2 years to 18 years 3.6–5.3 mmol/L
Chloride 0 day to <1 week 98–115 mmol/L
1 week – 18 years 98–110 mmol/L
Bicarbonate 0 day to <1 week 15–28 mmol/L
1 week to 2 years 16–29 mmol/L
2 years to 10 years 17–30 mmol/L
10 years to 18 years 20–32 mmol/L
Creatinine 0 day to <1 week 22–93 umol/L
1 week to 4 weeks 17–50 umol/L
4 weeks to 2 years 11–36 umol/L
2 years to 6 years 20–44 umol/L
6 years to 12 years 27–58 umol/L
12 years to 15 years 35–83 µmol/L
15 years to 19 years 50–100 µmol/L
19 years to 60 years 60–110 µmol/L
12 years to 15 years 35–74 µmol/L
15 years to 19 years 38–82 µmol/L
19 years to 60 years 45–90 µmol/L
Calcium 0 day to <1 week 1.85–2.80 mmol/L
1 week to 26 weeks 2.20–2.80 mmol/L
26 weeks to 2 years 2.20–2.70 mmol/L
2 years to 18 years 2.20–2.65 mmol/L
Phosphate 0 days to 1 week 1.25–2.85 mmol/L
1 week to 4 weeks 1.50–2.75 mmol/L
4 weeks to 26 weeks 1.45–2.50 mmol/L
26 weeks to 1 year 1.30–2.30 mmol/L
1 year to 4 years 1.10–2.20 mmol/L
4 years to 15 years 0.90–2.00 mmol/L
15 years to 18 years 0.80–1.85 mmol/L
18 years to 20 years 0.75–1.65 mmol/L
Magnesium 0 days to 1 week 0.60–1.00 mmol/L
1 week to 18 years 0.65–1.10 mmol/L
Alkaline phosphatase 0 days to 1 week 80–380 U/L
1 week to 4 weeks 120–550 U/L
4 weeks to 26 weeks 120–650 U/L
26 weeks to 2 years 120–450 U/L
2 years to 6 years 120–370 U/L
6 years to 10 years 120–440 U/L
10 years to 14 years 130–530 U/L
14 years to 15 years 105–480 U/L
15 years to 17 years 80–380 U/L
17 years to 19 years 50–220 U/L
19 years to 22 years 45–150 U/L
10 years to 13 years 100–460 U/L
13 years to 14 years 70–330 U/L
14 years to 15 years 50–280 U/L
15 years to 16 years 35–140 U/L
16 years to 22 years 30–110 U/L
 These so-called Harmonised Reference Intervals are also shown as part of the relevant Test information and under the heading: Reference Intervals or Ranges.

Harmonised intervals are a relatively new development and while there is agreement on the values of the actual interval, laboratories are in the process of adopting these intervals so it is possible that your results report may not show these intervals at the moment but in time all laboratories will adopt them for these particular tests.
More detailed information about Reference Intervals and how they are calculated can be found in Reference Intervals and What they mean.
Reference Intervals ( also known as Reference ranges) are a way of comparing your test results with those considered normal for the general population.
Last Review Date: February 1, 2023
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